Chemical tankers cargo operations are quite complex and potentially hazardous. Careful planning and preparations are essential during each of the following stages.
- Tank cleaning
PLANNING FOR LOADING
A great variety of cargoes are loaded aboard chemical tankers. The following should be considered while planning chemical cargo stowage.
The ship should be capable of carrying the cargo in accordance with the IBC Code for the Construction and Equipment of Ships Carrying Dangerous Chemical in bulk. For example a type III ship not accepts cargoes which may be carried by type I and II ship only.
CARGO TO BE CARRIED AND ANNEX II CATEGORIZATION
Many of the chemical cargoes are known by more than one name. The IMO Code for the Construction and Equipment of Chemical Tankers does not apply to certain types of cargoes while others must be carried strictly in accordance with the Code. The proper name of the cargo and its UN No as given in the IMO Code and Annex II must be identified so that the characteristics, hazards and other necessary information may be obtained for the purpose of safe handling and carriage.
The summary of minimum requirements for cargoes to which the IMO Code applies stipulates the type of containment or tank required for the carriage of various cargoes. This requirement should be complied with.
Because of their special characteristics very few chemical cargoes can be loaded in uncoated steel tanks. Most of the cargoes may be carried in stainless steel or stainless clad tanks. However, due to the high cost involved all chemical tankers may not have such stainless steel or stainless clad tanks. Chemical cargoes may be loaded in tanks coated with suitable coatings. Tanks coatings commonly used are.
- Zinc silicate
- Phenolic resins
- Rubberized coatings
Tank coating manufactures compatibility data should be consulted to ensure that the coating is suitable for the carriage of the intended cargo.
Many of the chemical cargoes are highly sensitive and are contaminated by impurities and/or by mixing with other cargoes and cargo residues. Some cargoes may react with one another in a hazardous manner. Care should be taken to ensure that separation is provided between such cargoes by means of,
- Void space
- Pump room
- Coffer dam
- Empty tank
Such cargoes should have separate pumping systems and venting systems. There are a number of compatibility charts published by various authorities. However, most authorities follow the US Coast Guard Compatibility charge.
When a chemical tanker is built its tanks are designed for a certain maximum load. A cargo which is likely to overstress any tank should not be given stowage in such a tank. If loading in cargo having a high specific gravity the quantity to be loaded should be restricted so as to avoid overstressing the tank/s.
Cargo is likely to expand due to a rise in temperature during the loaded passage. The officers concerned should calculate the filling ratios taking into consideration the maximum temperature that may be experienced during the passage.
Some chemical cargoes may require heating. Tanks with adequate heating arrangements should be allocated for the carriage of such cargoes. Care should be taken to ensure that the cargoes in the adjacent tanks will not be affected by such heating.
The officers planning the cargo stowage should bear in mind that cargoes may be contaminated by mixing of vapours in a common venting system. Similarly some cargo vapours may react with each other in a hazardous manner if allowed to mix with each other in the venting system. Care should be taken to segregate such cargo vapours.
The IMO Code specifies the gauging requirement in its summary of minimum requirements. A given cargo should not be loaded in a tank if it does not meet the gauging requirement. For example a cargo requiring closed gauging can not be loaded in a tank having a restricted or open gauging system.
GAS DETECTION REQUIREMENT
The summary of minimum requirement in the IBO Code specifies the type of gas detecting equipment required for each cargo. The officers concerned should ensure that the appropriate gas detectors/analyzers are available before deciding to load cargoes.
Different types of cargoes require different types of safety appliances such as breathing apparatus, protective clothing etc. The requirements as per the IMO Code should be complied with.
PUMPING AND PIPING ARRANGEMENTS
Pumping and piping arrangement vary from ship to ship. The ideal arrangement would be to have an individual pump and piping system for each cargo tank. However, due to the high cost involved all chemical tankers may not have such individual pumping and piping arrangement for each cargo tank. In ships using a common pump and piping system for a group of tank care should be taken while planning the cargo stowage and loading sequence to ensure that there is no contamination or chemical reaction due to incompatible cargoes mixing with each other in the pipelines.
STRESSES, BENDING MOMENTS AND SHEER FORCES
Cargo stowage and loading and deballsting sequence should be carefully planned so that the ship may not suffer undue stresses, bending moments or sheet forces during the entire loading operation. These stresses, moments and forces should be carefully calculated for various stages of loading. Loadicators and computers are highly useful in this respect.
TYPE OF INERT GAS AVAILABLE
Some cargoes are so sensitive to impurities that they may be contaminated by the impurities in inert flue gas. Before loading such cargoes the ship‘s officers should ensure that pure form of inert gas, normally Nitrogen, will be available in sufficient quantities. Some cargoes require pure nitrogen for inerting purpose.
DRAFT AND TRIM
The quantity of cargo to take and the loading sequence should be carefully planned so that the ship may not become aground at the berth due to excessive draft or trim.
PLANNING FOR DISCHARGE
While planning for cargo discharge the following should be taken into consideration:
Facilities available at the receiving terminal. Some terminals may not be able to handle a number of cargoes simultaneously. The rate of discharge and the tank sequence should be planned according to the capabilities and limitations of the receiving terminal. The terminal representatives must be consulted.
PUMPING AND PIPING ARRANGEMENT
In the case of ships which do not have individual pumps and pipelines for each cargo tank, care should be taken while planning the discharging sequence to ensure that there is no possibility of contamination or chemical reaction due to cargoes mixing with each other in the pipelines or pumps.